The present invention relates to a copier, a printer, a facsimile machine and an image forming apparatus using an electro photography method having the functions of the copier, the printer and facsimile machine. Particularly, the present invention relates to a color image forming apparatus including an intermediate transfer member for superimposing plural color toner images onto the intermediate transfer member to form an image.
In the electro photography method color image forming apparatus using the intermediate transfer member, known is an image forming apparatus arranged to transfer a toner image formed on an image carrier, which is a photoreceptor onto the intermediate transfer member (primary transfer), then the toner image on the intermediate transfer member is transferred onto a transfer material (secondary transfer). In this type of color image forming apparatus, the color image forming apparatus is designed to superimpose an electro-static toner image, which has been simultaneously formed on the image carrier with a predetermined polarity, onto the intermediate transfer member by using static electricity. Then static electricity transfers the toner image on the intermediate transfer member onto the transfer material at once.
The color image forming apparatus using the intermediate transfer member can superimpose the toner image formed on the image carrier onto the intermediate transfer member. Thus, the color image forming apparatus using the intermediate transfer member is widely applied to a color image forming apparatus. In this color image forming apparatus, after the toner image of each color formed on the image carrier is superimposed onto the intermediate transfer member, the superimposed toner images are transferred onto the transfer material at once by static electricity.
Since an electrostatic charge amount per a toner particle is substantially uniform, the toner layer voltage on the intermediate transfer member is determined by the toner adhesion amount in a predetermined area. In the color image forming apparatus, the electrostatic charge amount of the portion where toners of plural colors are superimposed among the toner images of the intermediate transfer member becomes larger than that of the portion where one color toner adheres. And for example, when there are a toner image of a solid portion and a toner image of a halftone portion on the intermediate transfer member, the electrostatic charge voltage of the solid portion is higher than that of halftone portion.
As described above, when toner image voltage dispersion on the intermediate transfer member is large, portions where transfer characteristics are different each other exist in the same toner image. When transferring all the portions where the transfer characteristics are different each other onto the transfer material under the same transfer condition, various poor quality images tend to appear when transferring the toner images from the intermediate transfer member to the transfer member.
In recent years, in the copier, the printer, the facsimile machine and the image forming apparatus such as a multifunctional product having the function thereof, the ratio of the machines having color capability has become high. At the same time, due to the adoption of polymerization toner and toner having a small diameter, the requirements for high quality images in a transfer process has become strong. Further, a high-speed process trend proceeds in the image forming apparatus. In response to these trends described above, in order to obtain a high quality image, it is necessary to correct the toner voltages on the intermediate transfer member, which vary according to the number of times of the first transfer and environment, so as to be substantially uniform, and to improve the second transfer performance.
Japanese Patent Application Publication No. 10-274892 discloses an image forming apparatus including a pre-transfer charging unit for charging a toner image onto an intermediate transfer member before conducting the second transfer to a transfer member.
Japanese Patent Application Publication No. 11-143255 discloses a potential difference controller to control a direct current voltage source of a secondary pre-transfer charging unit and a direct current voltage source of a secondary transfer device so that the difference between a toner image voltage of the secondary pre-transfer charging unit and a voltage of the second transfer device is substantially constant.
Japanese Patent Application Publication No. 06-236116 discloses an electro photography apparatus comprises a discharging unit for discharging a toner charge transferred onto an intermediate transfer member and a charging unit for charging the toner image on the intermediate transfer member with the same polarity when developing and to charge the toner image on the intermediate member with a reverse polarity against the charged polarity right before starting a second transfer.
In the color image forming apparatus for conducting the second transfer of a toner image from the intermediate transfer member to the transfer member after superimposing the toner image of each color formed on the surface of a photoreceptor onto the intermediate transfer member by using the first transfer unit, in order to prevent the occurrence of density unevenness due to the transfer charge deficit caused when the toner adhesive amount is large and the toner layer voltage is high, a secondary pre-transfer charging unit having a scorotron electrode is disposed on the upper stream of the second transfer unit to discharge the electrical charges of the toner image formed on the intermediate transfer member.
In this case, following is going to be a problem. Namely, when discharging the electrical charges of the toner image on the intermediate transfer member, the upper layer of the toner image turns to reverse electrical charge toner. As a result, floating toner adheres on a grid electrode having the same polarity of the toner before discharging the electrical charge of the toner image and the toner adhered on the grid deteriorates the dischargation control capability.
The color image forming apparatuses disclosed in Japanese Patent Application Publication No. 10-274892 and Japanese Patent Application Publication No. 11-143255 comprise a scorotron charging unit for charging electrical charges to form a toner image and removing the electrical charges, the scorotron charging unit being disposed in the upper stream of the second transfer unit. However, there is a possibility that floating toner adheres the grid electrode of the scorotron charging unit and deteriorates the control performance for charging the electrical charges.
The electro photography apparatus disclosed in Japanese Patent Application Publication No. 06-236116 is an apparatus wherein the electrical charges of a toner image is removed to zero by applying AC voltage, then toner image is recharged again. It is not an image forming apparatus in which a scorotron charging unit prevents the dirt of the grid in the scorotron charging unit. The charging unit disclosed here is a scorotron charging unit having no grid.